The focus of the Arizona Superfund Basic Research Program is to improve the risk assessment process through development of toxicologic and hydrogeologic models and to improve the remediation of federal and state superfund sites through development of novel in situ remediation technologies, particularly bioremediation technologies. The program examines these factors for two classes of commonly found chemicals: chlorinated hydrocarbons and metals. Interest in these two chemical classes was stimulated by two sites in Arizona which have been affected by them: (1) South Tucson Site - a superfund site under remediation for soil and groundwater containing trichloroethylene, dichloroethylene and chromium located in the southern part of Tucson and (2) Pinal Creek Site in Central Arizona - a State Department of Environmental Quality regulated site in central Arizona that is under evaluation for metals in groundwater, surface water and soil. Metals were leached from soils after mining activities. There is a mixture of chemicals at these sites, as there are at almost all superfund sites, and the problems posed by multiple chemicals form the basis for the final objective of the program: the risk assessment and remediation of chemical mixtures.